Scaling of atrioventricular transmission in mammalian species: an evolutionary riddle!

"Scaling deals with the structural and functional consequences of changes in size or scale among otherwise similar organisms." It plays a key role in all studies on comparative mammalian physiology and morphology. Heart weight is proportionally related to body weight and can be described by a straightforward, so-called allometric equation. We studied scaling of AV transmission times (PR intervals on the ECG) in 375 mammals of different dimensions and species. Scaling of AV transmission times versus heart length (third root of weight) is statistically best described by a S-shaped curve. This implies that AV transmission time in mammals is not linearly related to heart length and does not depend solely on the length of the AV transmission system. The AV node fine-tunes AV transmission times at rest and during exercise in individuals; it protects the ventricles against high-rate atrial arrhythmias such as atrial fibrillation; and it regulates basal AV transmission times in mammalian species of varying sizes. We call the "how" and "why" of the scaling of AV transmission time in mammals an evolutionary riddle that deserves further study.